The present invention relates to depositing patterned films of metals or metal oxides for use in a variety of applications such as the fabrication of microelectronic devices. The invention particularly relates to the photochemical deposition of metals or metal oxides from mesomorphous films containing precursor metal complexes.
Patterned films of metal oxides, metals or other metal containing compounds are vital to the microelectronics industry but also find applications as diverse as electrodes for capacitors, conductors, resistors or as diffusion barriers for minimizing the diffusion of a metal contact into a silicon substrate. There are a number of methods of making such films. Normally films of inorganic materials are deposited by chemical vapor deposition or physical vapor deposition although in some cases sol gel or metal organic deposition methods have been used. Since none of these methods is able to pattern films, additional technologies such as photoresists must be used to form the patterned structures employed in the construction of microelectronic devices or circuits.
Methods of patterning found in the prior art have a number of disadvantages. Generally, manufacturing techniques for such devices begin with a suitable substrate, often a semiconductor such as a wafer of crystalline silicon, upon which materials having the requisite electrical characteristics are deposited. Techniques that directly apply a metal film to a semiconductor by evaporation suffer from the drawback that the evaporation heats the substrate. The high temperature produced at the interface between the evaporated metal and the substrate causes metal atoms to diffuse into the substrate and vice-versa, resulting in a layer of mixed metal and semiconductor at the metal-semiconductor interface which can interfere with the performance of the device.
Other techniques are expensive because they generally include many steps. For example, when depositing material directly on a substrate, each patterning step typically involves: applying a photoresist to the surface of the substrate; changing the properties of selected areas of the photoresist by exposing those areas to light, X-rays or an electron or ion beam; removing either the exposed or unexposed portions of the photoresist to expose portions of the underlying substrate; chemically treating or depositing a material on the exposed portions of the substrate; and removing the photoresist. A further disadvantage of many such prior art techniques is that resolution can be lost in what is essentially a two-stage masking process.
An alternative photoresist technique involves depositing a layer of material on a substrate, applying a photoresist to areas where it is desirable to retain the material, then etching the material away in places where it is not required. However, this method has the disadvantages set out above and the further disadvantage that the edges of the retained material can be rough or undercut, a defect that can ultimately lead to cracks which can cause the entire device to fail.
Another disadvantage of prior art processes is that they tend to produce a non-planar surface because the materials are often not uniformly deposited over the surface of the substrate. If a generally planar surface is required then a separate planarization step such as with chemical mechanical polishing using an abrasive slurry is necessary.
As described in U.S. Pat. No. 4,952,556 to Mantese et al., patterns of superconducting material can be made by applying films of metallo-organic material on a substrate and patterning by irradiating with a beam of particles or electromagnetic radiation. When using light from a laser, an electron beam or an ion beam, patterning can be accomplished by local heating in the film and does not require a photochemical reaction. Local heating is disadvantageous because of local degradation of the substrate and because it is hard to control with precision.
Muller, U.S. Pat. No. 5,176,744, describes a method of depositing Copper from solutions of Copper formate, by irradiating with a laser beam. Although local heating is employed to decompose the copper compound, the judicious use of crystallization inhibiting agents ensures that the film of copper-containing solution has the consistency of a highly-viscous liquid and a more uniform deposition results.
Tutt and Duraiswamy, International Publication No., WO90/02827, disclose a method of photochemical deposition of high purity gold films in which a vapor of a gold containing complex photochemically decomposed while it is passed over a substrate. Although this technique avoids the disadvantages of thermal decomposition methods, it does not permit formation of patterned layers without the use of some other technology such as a photoresist or a mask.
In the photochemical deposition method (U.S. Pat. No., 5,534,312 to Hill et al., incorporated herein by reference) certain precursor metal complexes deposited on a silicon substrate can be caused to undergo photochemical reactions which result in the loss of the ligands associated with the metal complex. An amorphous film containing the precursor material is first applied to the substrate and is then subjected to a photochemical decomposition by irradiation with a beam of electromagnetic radiation, preferably visible or U/V light. The identity of the layer that results from the photochemical reaction can be controlled by the choice of precursor and atmosphere. For example, carrying out the photochemical reaction in air can lead to deposition of a metal oxide layer. Additionally, carrying out a first photochemical reaction in one atmosphere and a second photochemical reaction in a second atmosphere can permit patterns of two different types of material to be deposited on the substrate. The method differs from the above methods in that the reaction that creates the layer of metal, metal oxide or other metal compound is photochemically activated and in that the reaction takes place in a film on the surface of the substrate. In other prior art methods that use light as the energy source, the light initiates a thermal rather than a photochemical reaction. However, light-induced thermal reactions suffer from the drawbacks associated with local heating, as discussed above. The method also differs from previous methods in the art in that the film containing the metal complex is amorphous.
The success of the photochemical deposition method has spawned a desire to understand the properties of the precursor materials that led to the highest quality films and to a quest for additional classes of materials that are suitable, see for example, Hill, et al., Materials Chemistry and Physics, (1996), 43:233-237.
Consequently in one respect, it would be desirable to carry out photochemical deposition with a number of complexes that preferentially form films which are not amorphous but which have sufficient optical homogeneity for high quality lithographic applications.
The current invention describes the use of precursor metal complexes in mesomorphous, or generally liquid crystal, films, which can be converted to films of non-molecular metal containing materials by a variety of methods. Important is the utility of these films in the preparation of patterned films of metal containing materials on substrates by photolithographic methods. Previously it was thought that films of precursor materials were required to be amorphous to provide the optical properties necessary for high resolution optical lithography. It is now shown that partially ordered films may also provide the necessary optical conditions for lithography by photochemical metal organic deposition. The method can be used to make self-planar structures.
The present invention includes a method for making a pattern of a metal containing material on a substrate, the method comprising: (a) applying a mesomorphous film containing a metal complex on a surface of the substrate; (b) exposing, in a first atmosphere, a first area, having a first shape, of the film to electromagnetic radiation from a first source to cause the metal complex in the first area to undergo a photochemical reaction, the reaction transforming the metal complex in the first area into a first metal containing material adherent to the substrate and one or more ligand byproducts of a first kind at least some proportion of which are driven off during the course of the photochemical reaction, wherein the pattern comprises the first shape; (c) optionally driving off a remainder of the one or more ligand byproducts of a first kind that are not driven off during the course of said photochemical reaction. The method optionally also comprises, after the applying, (d) exposing, in a second atmosphere a second area, having a second shape, of the film to electromagnetic radiation from a second source to cause the metal complex in the second area to undergo a photo-chemical reaction, the reaction transforming the metal complex in the second area into a second metal containing material adherent to the substrate and one or more ligand byproducts of a second kind at least some proportion of which are driven off during the course of the photochemical reaction, wherein the pattern additionally comprises the second shape; and optionally (e) driving off a remainder of the one or more ligand byproducts of the second kind that are not driven off during the course of said photochemical reaction.
In one embodiment of the invention the selected atmosphere comprises oxygen and the metal containing material is a metal oxide. In another embodiment of the invention, a metal oxide produced according to the invention is reacted with a suitable chemical in a suitable atmosphere to reduce the metal oxide to a metal adherent to the substrate. Another aspect of the invention provides for the deposition of two different materials in a pattern. This aspect of the invention involves the steps of applying a mesomorphous film containing a metal complex on a surface of a substrate; placing the film in a first selected atmosphere; and exposing first selected areas of the film to electromagnetic radiation, which is preferably ultraviolet light, to cause the metal complex in the first selected areas to undergo a photo-chemical reaction. The reaction transforms the metal complex in the first selected areas into a first metal containing material adherent to the substrate. Subsequently, the film is placed in a second selected atmosphere; and second selected areas of the film are exposed to electromagnetic radiation to cause the metal complex in the second selected areas to undergo a photochemical reaction. The reaction transforms the metal complex in the second selected areas into volatile components and a second metal containing material adherent to the substrate.
The metal complex is preferably selected from the class of compounds known as metallomesogens. The metal complex comprises one or more metal atoms bonded to one or more ligands. In one aspect of the invention at least one ligand comprises an alkyl group. Preferably the ligands are selected from the group consisting of: carboxylates; pyridines; amines; diamines; arenes; alkoxy ligands; alkyl ligands; and aryl ligands. For applying metals, metal oxides and metal sulfides, the ligands are preferably small and do not comprise any organic groups containing more than 26 carbon atoms when one or more aryl groups is present. Most preferably, if the ligands do not include phenyl groups then the ligands do not comprise any organic groups containing more than 12 carbon atoms. In a preferred embodiment, the metal complex is a homonuclear dimetal complex. In another preferred embodiment, the ligand is a carboxylate, O2CR, wherein R is selected from C1-20 alkyl or C1-20 alkenyl or C1-20 alkynyl.
The invention has particular application in forming patterned films comprising metal oxides and/or metals. Other applications include the interconnection of components on semi-custom chips and the patterning of integrated circuits, either instead of or in conjunction with photoresist based patterning. The methods of the invention may be used in the manufacture of VLSI devices.